PHILCO PHORUM

The Phonograph Jack in Model 511 Series Philcos

Model 511 Series Philco receivers are unique in that they feature a 1/4 inch jack on the front panel, to the lower left of the tuning knob. The jack is described in Philco's Radio Manual of Useful Information, 1928-1929 as follows:

The following partial schematics illustrate its operation in the circuit:

As you see in the diagram below, point F provides B+ from the power supply circuit. This B+ has to pass through the phonograph jack. With no plug in the jack, the circuit from point F to the bottom of the audio interstage transformer primary is complete (between the arrow and the tip contact inside the jack), and the B+ is applied to the plate of the detector tube by way of the interstage primary. The arrow and tip contact act as a sort of switch; with no plug inside the jack, the "switch" is CLOSED.

Now see what happens when a 1/4 inch phone plug (illustrated in red) is inserted into the jack.

Now that there is a plug in the jack, the tip contact is raised up so that it no longer makes contact with the arrow; thus, the circuit is broken and B+ cannot make it to the plate of the detector tube. We can say that the "switch" inside the jack is now OPEN. Since the detector tube is not receiving any B+ voltage, the audio from the receiver front end is cut off. In addition, audio from the phono pickup is fed through this jack to the first audio interstage transformer (22), and is amplified by the audio tubes. The barrel of the plug/jack connection connects to the top end of the first audio interstage transformer (22), at the same point the plate of the detector tube is connected, as the tip end of the phone plug/jack connection is connected to the bottom end of the same audio interstage transformer.

This phonograph jack is only found on Model 511 Series radios. Philco filled the phono jack hole with a cover plug on Model 86 receivers, and it disappeared altogether after that.

BE CAREFUL if you need to remove this jack from the panel of a 511 Series radio. It is insulated from the metal case, using Bakelite washers for the purpose. Should you lose one of these Bakelite washers, the jack could become grounded to the case, resulting in a loss of B+ to the detector tube along with loss of reception. If you must remove this jack for any reason, be sure you reassemble in reverse order of your disassembly, using ALL of the bakelite washers!